These studies aim to characterize in detail the biochemical-genetic regulation of the essential folate-pathway enzyme in cultured mammalian, especially human, cells. The activities of 9 of the 15 major folate pathway enzymes have been detected in extracts of cultured skin fibroblasts, peripheral blood lymphoblasts and amniotic fluid cells. Activities of individual folate enzymes and of the pathway as a whole have been shown to be controlled in part by cell growth rate and density, and methionine synthetase activity is controlled in part by the composition of the medium, being derepressed on substitution of homocysteine for methionine. Skin fibroblasts cultured from patients with inborn errors of folate metabolism are being used to understand the biochemical changes in methylenetetrahydrofolate reductase deficiency. Related disorders that cause mental retardation and/or neurologic abnormalities are under study in cell culture and include dihydropteridine reductase deficiency and the fragile X chromosome syndrome.